The present invention is directed to the compounding of plastic pellets, where one or more plastic resins and one or more additives are melted and mixed in a compounding extruder. In the present methods of compounding and forming plastic pellets, the heat generated in cooling the extrudate that forms the pellets is thrown away. The prior art method of forming plastic pellets is illustrated in the schematic drawings labeled FIG. A and FIG. B.
Referring to FIG. A, extruder 10 is a single screw or double screw compounding extruder powered preferably by a DC motor 12 and a gear drive 14. Extruder 10 has a plurality of zones, seven (7) being shown. The zones are heated by heaters (not shown) on the external surface of extruder 10. A feed hopper 16 is located at the near end of extruder 10, near the first of the internal zones, usually near the first or second zone. The temperature within the extruder 10 usually increases as the plastic resin and additives advance from the first to last zone, and the plastic resin that is introduced as a solid is melted to a point that the additive(s) are homogeneously mixed in the plastic resin. Extruder 10 has a die 18 having a plurality of holes through which the mixed plastic and additive(s) flow as an extrudate. Above the feeder hopper 16 of extruder 10 is shown feed handling equipment 20, the details of which are immaterial to the description of the prior art. Extruder 10 has a pelletizer device 30 at the terminal end. Illustrated is an underwater pelletizer characterized by structure that brings cool water from inlet “A” through a chamber with a rotating cutter that cuts the extrudate immediately as the extrudate passes through die 18, cooling the cut extrudate as a pellet and removes the quench water, that is now hot, and the pellets through outlet “B”.
Referring now to FIG. B, the slurry of pellets in the quench water from extruder 10 and pelletizer 30 enter through “B” into equipment 40 for pellet recovery. The pellets are separated from the water in a de-watering box 42 and the water collected in water tank 44. The water exits water tank 44 and is passed through a heat exchanger 46, which may be a chiller or cooling towers, to reduce the temperature of the water. In the prior art, this heat is essentially thrown away. The heat must be removed since the water is re-circulated, after passing though filters 48 to exit “A” for use in the pelletized 30 as shown in FIG. A.